Divergent Prey Selection in Two Species of Waterthrushes (Seiurus)

Abstract

Abstract I studied Louisiana (Seiurus motacilla) and Northern (S. noveboracensis) waterthrushes during an exceptionally dry spring to determine if environmental stress elicited interspecific competition. Previously, I had found little evidence for competition between these species despite wide overlap in foraging methods, use of foraging microhabitats, and characteristics of breeding habitat. I observed breeding adults forage by placing them in a portable flight cage located in natural habitat, and concurrently gathered data on the influence of waterthrush foraging on aquatic invertebrate prey and prey abundance. The species selected different prey. Louisiana Waterthrushes fed predominantly on Trichoptera larvae and on larger average prey than did Northern Waterthrushes, which fed predominantly on Diptera larvae. The species had similar foraging methods and attack rates, indicating that, unlike many Parulinae, their principal foraging differences were in prey selection rather than in means of locating prey. Experiments with foraging exclosures gave no clear evidence that waterthrushes affected prey biomass or composition. There was no significant relation between territory size vs. prey biomass and water cover. No interspecific aggression was observed, and territories overlapped widely, indicating that interference competition did not occur. Divergence in prey selection implies exploitive competition, but invertebrate and habitat data indicate that prey were not limiting, thus making competition for prey unnecessary. Therefore, from these data I cannot eliminate the alternative hypothesis that observed differences between the species may only reflect independent specialization.